McRaney is asking you to believe your judgements are often guided by irrationality rather than by reason and that you do not always understand why you make the decisions you do. For example, you have preconceptions, biases, and shortcuts in your thinking (called heuristics) that make you prone to errors of judgment and decision making. An example is priming, when a stimulus from the past affects your current decision. In a study cited by the author, subjects were asked to remember a sinful memory. Half of the participants washed their hands, while others did not. Those who did not were more likely to agree at the end of the study to help a graduate student for no money, and the researchers posited that it was because the people who had washed their hands had unconsciously washed away their guilt. This study is an example of how the unconscious plays a powerful role in our decision making.
The premise of the book might challenge your ideas that your decisions are always guided by logic and reason, or that you even know why you choose to make the decisions you do. Many people are surprised when they realize that illogic, the unconscious, and other forces beyond their control affect their decisions in powerful ways and that cognition is not entirely rational and conscious.